The multiple roles of chlorite on the concentrations of radicals and ozone and formation of chlorate during UV photolysis of free chlorine

Chlorine dioxide (ClO2) has emerged as a promising alternative to free chlorine for water disinfection and/or pre-oxidation due to its reduced yields of chlorinated disinfection byproducts. ClO2 decomposes to form chlorite (ClO2-), which influences the following advanced oxidation processes (AOPs) for micropollutant abatement in drinking water. This study aims at investigating the effects of ClO2- on the concentrations of reactive species (e.g., radicals and ozone) and on the formation of chlorate in the UV/chlorine AOP. Results showed that the concentration of ClO center dot in the UV/chlorine process remarkably decreased by 98.20-10 0.00% in the presence of ClO2- at concentration of 0.1-1.0 mg.L-1 as NaClO2. The concentrations of HO center dot and ozone decreased by 42.71-65.42% and by 22.02-64.31%, respectively, while the concentration of Cl-center dot was less affected (i.e., 31.00-36.21% reduction). The overall concentrations of the reactive species were differentially impacted by ClO2-'s multiple roles in the process. UV photolysis of ClO2- generated HO center dot but not Cl-center dot, ClO center dot or ozone under the drinking water relevant conditions. ClO2- also competed with chlorine for UV photons but this effect was minor (< 1.0%). The radicals/ozone scavenging by ClO2- out-competed the above two to lead to the overall decreasing concentrations of the reactive species, in consistency with the kinetic model predicted trends. ClO2- reacted with radicals and ozone to form chlorate (ClO3-) but not perchlorate (ClO4-). HO center dot played a dominant role in ClO3- formation. The findings improved the fundamental understanding on micropollutant abatement and inorganic byproduct formation by the UV/chlorine process and other AOPs in ClO2--containing water. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The presence of ClO2- in the UV/chlorine AOP significantly decreases the concentrations of reactive species, leading to an impact on chlorate formation. ClO2- reacts with radicals and ozone, reducing the overall concentrations of reactive species.